[Aura] Support always-on-top top level window.

ui/aura_shell/shell.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura_shell/shell.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "ui/aura/aura_switches.h"
 #include "ui/aura/desktop.h"
@@ skipping 74 matching lines @@
 Shell::Shell(ShellDelegate* delegate)
     : ALLOW_THIS_IN_INITIALIZER_LIST(method_factory_(this)),
       delegate_(delegate) {
   aura::Desktop::GetInstance()->SetEventFilter(
       new internal::DesktopEventFilter);
   aura::Desktop::GetInstance()->SetStackingClient(
       new internal::StackingController);
 }
 
 Shell::~Shell() {
+  DCHECK(instance_ == this);
+  instance_ = NULL;
+
   // Make sure we delete WorkspaceController before launcher is
   // deleted as it has a reference to launcher model.
   workspace_controller_.reset();
 }
 
 // static
 Shell* Shell::CreateInstance(ShellDelegate* delegate) {
   CHECK(!instance_);
   instance_ = new Shell(delegate);
   instance_->Init();
@@ skipping 18 matching lines @@
 
   aura::Window::Windows containers;
   CreateSpecialContainers(&containers);
   aura::Window::Windows::const_iterator i;
   for (i = containers.begin(); i != containers.end(); ++i) {
     (*i)->Init(ui::Layer::LAYER_HAS_NO_TEXTURE);
     desktop_window->AddChild(*i);
     (*i)->Show();
   }
 
+  internal::StackingController* stacking_controller =
+      static_cast<internal::StackingController*>(
+          desktop_window->stacking_client());
+  stacking_controller->Init();
+
   internal::DesktopLayoutManager* desktop_layout =
       new internal::DesktopLayoutManager(desktop_window);
   desktop_window->SetLayoutManager(desktop_layout);
 
   desktop_layout->set_background_widget(internal::CreateDesktopBackground());
   aura::Window* default_container =
       GetContainer(internal::kShellWindowId_DefaultContainer);
   launcher_.reset(new Launcher(default_container));
 
   views::Widget* status_widget = NULL;
@@ skipping 45 matching lines @@
       new internal::WorkspaceController(default_container));
   workspace_controller_->SetLauncherModel(launcher_->model());
   default_container->SetEventFilter(
       new internal::DefaultContainerEventFilter(default_container));
   default_container->SetLayoutManager(
       new internal::DefaultContainerLayoutManager(
           workspace_controller_->workspace_manager()));
 }
 
 }  // namespace aura_shell